1. Field of the Invention
The present invention relates to a control apparatus for controlling the output of a control object.
2. Description of the Prior Art
A known control apparatus of the type mentioned above is disclosed, for example, in Laid-open Japanese Patent Application No. 2001-227385. This control apparatus controls the opening of a throttle valve of an internal combustion engine which is intended for the output of a control object. The opening of the throttle valve (hereinafter simply called the “throttle opening”) is determined by the balance of an urging force generated by a return spring which urges the throttle valve in a closing direction at all times and a driving force generated by a motor which is connected to the throttle valve. Also, the control apparatus calculates a control input to the motor in the following manner to control the throttle opening such that it converges to a target opening. Specifically, the control apparatus first calculates a preliminary value for the control input based on an actual throttle opening and a target opening in accordance with a sliding mode control algorithm. Then, in consideration of hysteresis of the return spring, the control apparatus calculates the control input to the motor by adding a predetermined value commensurate with the hysteresis to the calculated preliminary value. In this way, the control apparatus controls the throttle opening with high accuracy in accordance with the hysteresis of the return spring.
However, when the conventional control apparatus described above is applied to a control object which presents low stability, the following problems may arise. Specifically, in a control system which includes a control object that presents low stability, when the output of the control object is controlled to reach a target value, the control object tends to suffer from fluctuations in the output when periodic disturbance acts thereon, and from a large instantaneous deviation when large disturbance instantaneously acts thereon, respectively. In response to such a behavior, the conventional control apparatus merely adds a predetermined value commensurate with the hysteresis of the return spring, so that fluctuations or an instantaneous large deviation is highly likely to appear in the output. Particularly, the sliding mode control has a high responsibility of the output to a target value, so that if the output fluctuates at high frequencies due to the influence of periodic high-frequency disturbance, the control input can resonate with the fluctuations, thereby possibly causing a further increase in the deviation. As described above, the conventional control apparatus is incapable of providing high stability and control accuracy for the output of the control object.